Method for polishing gallium arsenide



Nov. 10, 1964 M. v. SULLIVAN 3,155,596

METHOD FOR POLISHING GALLIUM ARSENIDE Filed Dec- 29. 1961 FIG.

ROTATING MECHANISM L F .l n ,ILL "JA- l u/ /a I 1 I I LU FIG. 2

. INVENTOR M. l SULLIVAN EAM m 1 1% A T TORA/E Y United States Patent3,156,596 METHOD FOR POLISHING GALLIUM ARSENIDE Miles V. Sullivan,Summit, N.J., assignor to Bell Telephone Laboratories, Incorporated, NewYork, N.Y., a corporation of New York Filed Dec. 29, 1961, Ser. No.163,126 Claims. (Cl. 156-17) This invention relates to a method forpolishing gallium arsenide.

More specifically, this invention is directed to a technique forpolishing crystals of gallium arsenide with a solution comprising amixture of bromine and methanol, such solution being particularly wellsuited as a substitute for or supplement to the usual mechanicalpolishing procedure for these crystals.

In the fabrication of semiconductor devices it is essential to preparethe (semiconductor) starting material in slices which have flat, smooth,damage-free surfaces. This is particularly critical for diffused devicessince imperfect surfaces disturb the even passage of the diffusant intothe semiconductor slices. This, in turn, deleteriously affects theelectrical characteristics of the device.

Several techniques such as electropolishing, chemical etching andmechanical lapping of semiconductor slices are well known and presentlyin use in the semiconductor art. By lapping and polishing techniques asurface can be prepared which is about 0.0001 inch per inch flat. Theaverage roughness of this surface is typically 0.3 microinch. Theresultant surfaces are smooth and fiat enough for satisfactory devicefabrication but the mechanical damage which is still present on thesurface deleteriously affects the electrical characteristics of devices.Typically, it is necessary to remove such damaged surfaces prior tofurther processing by chemically etching the surface. Unfortunately,chemical etching with conventional etchants, such as aqua regia ormixtures of nitric and hydro fluoric acids, increases the averageroughness of a typical surface to about 3.0 microinches or greater.Thus, the gain in removing mechanical damage by the chemical etchingtechnique is obtained at the expense of increasing the averageroughness. In order to obtain a surface which is suflicientlydamage-free and still maintain a low average roughness and suitableflatness, these processes, namely, mechanical polishing and chemicaletching, may be alternated several times, however, resulting inincreased production costs.

In accordance with this invention a technique is described for polishingthe surfaces of gallium arsenide crystals with a bromine-containingmethanol solution, thereby producing surfaces which are smooth to betterthan 0.5 microinch, flat to better than 0.0001 inch per inch and free ofmechanical damage. As described herein, etching is conducted inconjunction with a mechanical stirring mechanism resulting in agitationproximate the surface to be etched.

In an illustrative embodiment the inventive technique involves mountingthe gallium arsenide on a rotatable disk such that the surface ofinterest is exposed. A second rotatable disk optionally covered by alow-pile cloth is positioned proximate the surface of interest such thatthe pile of the cloth is almost in contact with such surface. The seconddisk is rotated about an axis of rotation parallel to but not coincidentwith that about which the first disk is rotated. The bromine-containingmethanol solution is provided to a level above the pile of the cloth forcontacting the surface to be polished.

The invention will be more fully understood from the following detaileddescription taken in conjunction with the following drawing wherein:

FIG. 1 is a perspective view, partially in cross-section,

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of an apparatus suitable for the present inventive purposes; and

FIG. 2 is an enlarged view, partially in cross-section, of a portion ofthe surface of a gallium arsenide crystal and the stirring mechanism ofthe apparatus employed.

Referring more particularly to FIG.l, there is shown a cylindricalcontainer 11 having a centrally located rotating mechanism 12, typicallyan electric motor, secured to the container by supports 13. A metallicshaft 14 transmits angular motion from the rotating mechanism to a.glass stirring disk 15 positioned immediately below the annular lip 16of the cylindrical container 11. The glass stirring disk is covered by alow-pile cloth 17. The diskshaped block 18 is secured to a holder 20 bya suitable connection which allows block 18 to rotate about an axis ofrotation 22 which is parallel to but eccentrically disposed to shaft 14.A plurality of gallium arsenide slices 23 are aflixed to the surface 24of block 18 such that the surfaces 25 of slices 23 are in contact withthe pile 26 of the low-pile cloth 17 which may also be a material suchas Dextilose paper which is obtained commercially from the Dexter PaperCompany. The term low-pile refers to any cloth or high wet strengthpaper having a pile of height of approximately 0.030 inch or less. Tube28 provides a continuous supply of solution. The level of the solutionis kept sufficiently high that the surface of interest is wet by it.

In operation, slices 23 are made to rotate independently of stirringdisk 15, or are dragged by the rotation of the disk. In either instance,the result is a continuously changing direction of polishing for slices23 because of the eccentric arrangement of the axis of rotation 22 andthe shaft 14.

The presence of a source of radiation having a Wavelength in the visiblerange, such as a 300-600 watt tungsten filament light bulb has atendency to reduce preferential etching in heavily damaged regions,thereby resulting in smoother gallium arsenide surfaces. To this end,radiation source 29 is suitably positioned in the apparatus shown inFIG. 1 proximate glass disk 15. It will be appreciated by those skilledin the art that the exact location of such source may be varied asdesired.

The solution employed in the present technique comprises methanol andbromine wherein the bromine is present in an amount within the range of0.001 to 0.05 percent by volume of the total solution. The use ofconcentrations beyond 0.05 percent adversely alfects the polishing inthat pitting in the gallium arsenide slice is increased. The lower limitof 0.001 percent of bromine by volume of the total solution is notabsolute and is dictated by considerations of etching rate.

The etching solutions herein contain a maximum bromine content of 0.0025percent and are suitable for polishing all faces of gallium arsenideincluding the (m) and (111) and such range is preferred for the practiceof this invention. However, higher concentrations up to 0.05 percentwhich are unsuitable on the (111) face are useful and often desirablefor polishing the other faces of this crystalline system.

FIG. 2 shows on an expanded scale the spatial relations of the variouselements for optimum operation.

In particular, the spatial relations are such that the pile 34 isseparated advantageously from the high points 36 of the surface adistance 33 which is of the same order as magnitude, preferably betweenthree times and a third, as the depth 38 of an average depression belowthe surface. However, it is tolerable if the pile makes grazing contactwith the high points of the surface.

Experimentation has shown that by maintaining the stirring means at sucha distance, the etch rate at the high points can be increased by atleast a factor of ten. This Q2 results in gallium arsenide surfaceswhich are smooth to within 0.05 microinch and flat to within 0.0001 inchper inch over a square centimeter of surface area.

Several examples of the present invention are described in detail below.These examples and the illustration described above are included merelyto aid in the understanding of the invention, and variations may be madeby one skilled in the art without departing from the spirit and scope ofthe invention.

Example I Three slices of (TE) gallium arsenide, approximately 0.5 inchby 0.5 inch by 0.02 inch lapped with 1950 mesh alumina were affixed tothe surface of block 18 in the apparatus shown in FIG. 1. Disk 15 was an8 inch glass cylinder having Dextilose paper attached to one surfacethereof. A bromine-methanol solution containing 0.05 percent bromine byvolume of solution was admitted to container 11 in an amount sufficientto wet the surfaces of the gallium arsenide. Disk 15 was rotated at 72revolutions per minute while illuminated by a 500 watt tungstenfilament. After 25 minutes the thickness of the slices was reduced fromapproximately 0.02 to 0.019 inch, the smoothness of the surfaces changedfrom 4.0 to 0.1 microinch and the flatness was unchanged from itsoriginal 0.0001 inch per inch on all but the outer 10 percent of thesurface.

Example II Three slices of (111) gallium arsenide, approximately 0.5inch by inch by 0.02 inch were lapped with 1950 mesh alumina and thenwith 0.3 micron alumina and were affixed to the surface of block 18 inthe apparatus shown in FIG. 1. The procedure of Example I was nextrepeated with the exception that a bromine-methanol solutioncontaining0.0025 percent bromine by volume of total solution was employed. After40 minutes the thickness of the slices was reduced from approximately0.02 to 0.0199 inch, the smoothness of the slice changed from 0.5 to 0.1microinch and the flatness was unchanged from its original 0.0001 inchper inch on all but the outer percent of the surface.

While the invention has been described in detail in the foregoing.specification, it will be appreciated by those skilled in the art thatvarious modifications may be made 4 without departing from the spiritand scope of the invention, reference being had to the appended claims.

What is claimed is:

1. The method of polishing a crystal of gallium arsenide which comprisesthe steps of wetting said crystal with a mixture consisting essentiallyof bromine and methanol, said bromine being present in an amount withinthe range of 0.001 to 0.05 percent by volume of the total solution andstirring along a plane substantially parallel to and at a distance fromthe high points of said crystal of the order of the depth of thediscontinuities of said crystals.

2. The method of claim 1 wherein said stirring means is a glass disk.

3. The method of claim 2 wherein said crystal is illuminated.

4. The method of claim 1 wherein said stirring means is a low-pile clothcovered glass disk.

5.v The method of claim 4 wherein the distance between said crystal andsaid cloth covered disk is less than 0.030 mil.

6. The method of polishing the (111) face of gallium arsenide whichcomprises the steps of wetting said (111) face with a mixture consistingessentially of bromine and methanol, said bromine being present in anamount within the range of 0.001 to 0.0025 percent by volume of thetotal solution and stirring along a plane substantially parallel to andat a distance from the high points of said (111) face of the order ofthe depth of the discontinuities of said (111) face.

7. The method of claim 6 wherein said stirring means is a glass disk.

8. The method of claim 7 wherein said crystal is illuminated.

9. The method of claim 6 wherein said stirring means is a low-pile clothcovered glass disk.

10. The method of claim 9 wherein the distance between said crystal andsaid cloth covered disk is less than 0.030 mil.

References Cited in the file of this patent UNITED STATES PATENTS2,539,455 Mazia Jan. 30, 1951 2,841,477 Hall July 1, 1958 3,024,148Schaer Mar. 6, 1962

1. THE METHOD OF POLISHING A CRYSTAL OF GALLIUM ARSENIDE WHICH COMPRISESTHE STEPS OF WETTING SAID CRYSTAL WITH A MIXTURE CONSISTING ESSENTIALLYOF BROMINE AND METHANOL, SAID BROMINE BEING PRESENT IN AN AMOUNT WITHINTHE RANGE OF 0.001 TO 0.05 PERCENT BY VOLUME OF THE TOTAL SOLUTION ANDSTIRRING ALONG A PLANE SUBSTANTIALLY PARALLEL TO AND AT A DISTANCE FROMTHE HIGH POINTS OF SAID CRYSTAL OF THE ORDER OF THE DEPTH OF THEDISCONTINUITIES OF SAID CRYSTALS.